1. Field of the Invention
The present invention relates to a switching power-supply device including a converter with a two-stage configuration.
2. Description of the Related Art
In Japanese Unexamined Patent Application Publication No. 64-43062, a DC-DC converter is disclosed that has a configuration in which a current-input-type converter is provided in a previous stage and a series resonant converter is provided in a subsequent stage. The current-input-type converter located at the previous stage detects an output voltage, and controls an input voltage to the series resonant converter located at the subsequent stage so as to have a given level. The series resonant converter located at the subsequent stage operates with a fixed frequency so that the input voltage becomes a load voltage that does not change.
In Japanese Unexamined Patent Application Publication No. 64-43062, the current-input-type converter located at the previous stage includes a capacitor in the output thereof. If the charging voltage of this capacitor is Vc and the input voltage thereof is Vs, a relationship with an input voltage Vi to the converter located at the subsequent stage becomes Vi=Vs−Vc. In Japanese Unexamined Patent Application Publication No. 64-43062, when an instantaneous power failure is taken into consideration, it may be necessary to make a voltage conversion ratio large, which is due to the current-input-type converter located at the previous stage.
For example, when the input voltage Vi of the converter located at the subsequent stage is about 200 V, the charging voltage Vc of the capacitor is reduced at the time of the instantaneous power failure, and thus, the DC-DC converter is operable until the input voltage Vs becomes reduced to about 200 V. On the other hand, when it is assumed that the input voltage Vs at the time of a steady state is about 380 V, it may be necessary for the current-input-type converter to perform a voltage conversion operation so that the charging voltage Vc of the capacitor becomes about 180 V. In this manner, in the DC-DC converter described in Japanese Unexamined Patent Application Publication No. 64-43062, when an input voltage range is large in view of the instantaneous power failure, a problem has occurred in which the voltage conversion ratio at the time of a steady state is large and inefficiency arises.